A Perspective on Four New Porphyrin-Based Functional...

CONTENTS

243A perspective on four new porphyrin-based functional materials and devicesCharles Michael Drain*, Joeseph T. Hupp, Kenneth S. Suslick, Michael R. Wasielewski, Xin Chen

A perspective on the formation and use of porphyrins as materials and as components of devices is given. Self-assembled, self-organized, and covalent porphyrin arrays for applications as sensors, sieves, catalyts and photonic devices are discussed as is arrays of metalloporphyrins and other dyes as cross-reactive sensors.

259Recent developments in the coordination chemistry of porphyrin complexes containing non-metallic and semi-metallic elementsPenelope J. Brothers*

Recent advances in the chemistry of main group porphyrin complexes are surveyed, including boron and tellurium porphyrins which reveal unprecedented structural types, advances in the preparation and reactivity of Group 14 (silicon and tin) and Group 15 porphyrin complexes, and a discussion of out-of-plane distortions (ruffling) of light ele-ment Group 14 and 15 porphyrin complexes.

268Overview of optical sensors using porphyrinsIchiro Okura*

The background and the concept of optical sensor technology are briefly introduced and a new optical sensing system by triplet-triplet absorption is discussed as an example.

271From studies on artemisinin derivatives to trioxaquinesBernard Meunier*

Artemisinin, an antimalarial trioxane is able to alkylate the heme after activation by heme itself via an electron transfer from the iron(II) to the endoperoxide function of the drug. Based on these mechanistic studies, new antimalarial agents (trioxaquines) have been synthetized.

Journal of Porphyrins and PhthalocyaninesJ. Porphyrins Phthalocyanines 2002; 6: 241-304

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Artemisinin

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CONTENTS

J. Porphyrins Phthalocyanines 2002; 6: 241-304

274Synthetic advances in phthalocyanine chemistryGema de la Torre and Tomás Torres*

This overview is devoted to present the state-of-the-art in the preparation of low-sym-metry phthalocyanine derivatives and analogues. New synthetic trends focused on the preparation of phthalocyanines and Pc analogues bound to other electronically active molecules are also treated.

285Recent advances in the electrochemistry of porphyrins and phthalocyaninesFrancis D’Souza*

An overview of the recent developments in the electrochemistry of metalloporphyrins and metallophthalocyanines is presented by focusing briefly on the electroanalytical, electrocatalytic and electrochemical sensor applications.

289Electron transfer in electron donor-acceptor ensembles containing porphyrins and metalloporphyrinsDirk M. Guldi* and Shunichi Fukuzumi

The function of porphyrins and metalloporphyrins in novel, redox- and lightactive donor-acceptor ensembles is critically reviewed. Besides ac-ting as light harvesting antenna moieties, porphyrins and metalloporphy-rins show particular benefits when used as electron donor components.

296Theoretical aspects of the spectroscopy of porphyrins and phthalocyaninesMartin Stillman*, John Mack, and Nagao Kobayashi*

The development of theoretical models that describe the electronic structures of the porphyrins and phthalocyanines using ZINDO and DFT techniques is rapidly approaching the point where the major spectral bands over a wide energy range, for both symmetric and non−symmetric molecules can be fully accounted for.

301Resonance Raman spectroscopyTeizo Kitagawa*

A brief overview of topics and papers given in Kyoto at the ICPP-2 microsymposium on resonance Raman spectroscopy is presented.

A Perspective on Four New Porphyrin-Based Functional Materials and Devices

Charles Michael Drain,*a Joeseph T. Huppb, Kenneth S. Suslick,c Michael R. Wasielewski,b Xin Chena

aDepartment of Chemistry & Biochemistry, Hunter College of the City University of New York, 695 Park Avenue,

New York, NY 10021

bDepartment of Chemistry, Northwestern University, 2145 Sheridan Road, Evenston, IL 60208

cDepartment of Chemistry, University of Illinois, 600 S. Mathews Avenue,

Urbana-Chamgaign, IL 61801

Received date (to be automatically inserted after your manuscript is submitted)

Accepted date (to be automatically inserted after your manuscript is accepted)

ABSTRACT: The tremendous potential for the manifold applications of porphyrins, porphyrazines,

and phthalocyanines derives from their photophysical and electrochemical properties, their remarkable

stability, and their predictable and rigid structure. These applications include nonlinear optics,

catalysts, sensors, actuators, molecular sieves, and therapeutics. All of these properties are modulated

by appending various chemical moieties onto the macrocycles, by choice of metallo derivative, and by

the choice of environment. In multichromophoric systems, furthermore, the relative orientation of the

chromophores, the nature of the linker, and the size of the system also dictate the properties. The

synthesis of multichromophoric systems – both via covalent and noncovalent linkers – is driven by the

desire to make new materials and to understand biological processes such as the various aspects of

photosynthesis. Though electron and energy transfer processes continue to drive the synthesis of ever

more complex systems, more recent focus has shifted toward other applications and functionalities of

these structures. The focus of this perspective is on four recent developments in formation and

characterization functional, porphyrinic materials and devices: (1) self-assembly and self-organization

of porphyrin arrays and aggregates into phototransistors and photonic devices; (2) self-assembled

porphyrin squares for sensors, sieves, and catalysts; (3) spatially separated arrays of metalloporphyrins

as stochastic sensors; and (4) covalently bound arrays of different chromophores as photonic

materials.

KEYWORDS: porphyrins, self-assembly, photonics, catalysts, sensors, arrays

*Correspondence to: Charles Michael Drain, e-mail :[email protected]

INTRODUCTION

The potential applications of porphyrins in the design of functional materials, sensors, catalysts, and sieves are

well documented [1-6]. The rich photo chemistry and redox chemistry is readily modulated by both the natures of

the substituents on the macrocycle and of the metallo derivatives. Beyond simple environmental factors that effect

any chromophore, further modulation of the chemistry is accomplished by the relative order imposed in

supramolecular arrays, crystals, and aggregates formed by self-assembly and self-organization. The nature of the

intermolecular interactions (covalent, hydrogen bond, coordination, electrostatic, hydrophobic, etc.) used to form

both discrete and polymeric multi-porphyrin systems also helps to dictate the properties, and in the case of structures

mediated by coordination chemistry the metal ion linker can add funtionality. Stochastic sensors, actuators, and

screens using array-based technologies – in the present case using wells of individual metalloporphyrins distributed

in arrays on a solid support – is thematically related in that it is the response of the ensemble that provides the

enhanced function. This perspective focuses on the recent advances, since 2000, in the applications of porphyrin-

based materials from the labs for the four authors.

SELF-ASSEMBLY AND SELF-ORGANIZATION OF PORPHYRIN ARRAYS,

AGGREGATES AND DEVICES

Self-assembly and self-organization are widely believed to be central issues to the utilization of nanoscaled

materials in real-world fabrication and application of devices based technologies and/or functions exploiting this

scale[7]. Conceptually, self-assembly is used to design and form structures wherein the function allows little

tolerance for defects and generally uses specific intermolecular interactions to form specific, discrete,

supramolecular structures albeit with varying yields. Self-organization, on the other hand, is used when there is

more tolerance for defects and generally relies on a combination of non-specific interactions. The inherent

redundancies in the latter mitigate the defects, but high quality crystals needed for some applications are a notable

exception. In these regards one may look upon the organization and structure of mater into functional nanoscaled

materials as falling into four broad motifs. 1. The primary structure is molecular structure – of which we have

exquisite control by the highly developed principles of organic chemistry. 2. The secondary structure is

supramolecular structure – again a well-developed field based on well-understood principles. 3. The tertiary

structure describes how the supermolecules interact to form a higher ordered material that can be aggregate or

crystalline – though significant progress has been made to design and predict hierarchical structure, this area is still

developing. 4. The quaternary structure describes how the self-assembled and/or self-organized material self-

incorporates into the device or devices and may include the interconnects to the macroscopic world. The quaternary

structure is the least developed and the least studied aspect of nanoscaled porphyrinic materials [7].

The tremendous progress in synthetic supramolecular chemistry has yielded a large number of systems wherein

two or more porphyrins are self-assembled via a variety of intermolecular forces [1-6]. Both the greater stability and

the greater design flexibility afforded by coordination chemistry to self-assemble both discrete and polymeric

porphyrinic materials has been exploited to make and characterize a variety of functional materials. On the other

hand, the use of complementary hydrogen bonding moieties to stitch together porphyrins has advantages when the

self-assembled porphyrin oligomer needs to adapt to its environment in order to self-organize into a functional

device. Self-assembled systems using these two specific, directional intermolecular forces can be self-assembled

into discrete hierarchical structures via secondary supramolecular synthetic steps, or can self-organize into

hierarchical aggregates via non-specific, non-directional interactions such as van der Walls, electrostatic, and

hydrophobic. In terms of discrete supramolecular species, both coordination chemistry and hydrogen bonding have

been used to make square tetrameric arrays of porphyrins [8-11].

The addition of 12 Pd(II) with labile ligands in the trans position to four ‘L’-topic porhyrins which serves as

corners, four ‘T’-topic porphyrins which serves as the sides, and one ‘+’ topic porphyrin which resides in the center,

results in the self-assembly of a planar 3x3 nonamer [12]. The efficiency of self-assembly of these 21 chemical

entities is qualitatively the same for both the free base and several metalloporphyrin derivatives. Recent results

indicate that the nonamer can be metalated in situ by the addition of 9 equivalents of one of several first-row

transition metats, which represents the self-assembly of a 30-particle system, Figure 1 [13]! The kinetics of self-

assembly the metallononamer and subsequent self-organization of the aggregates are significantly slower, likely

because the greatly increased number of possible intermolecular interactions (such as pyridyl coordination to the

metal ion, or to a metalloporphyrin), but works because of well understood complexation energetics and kinetics for

both the first row metals and palladium. In a second supramolecular synthetic step, a dimer of the 3x3 arrays is

nearly quantitatively afforded by the addition of nine equivalents of 4,4’-bipyridine to the all-zinc porphyrin

nonamer (before aggregation) to form a sandwich wherein all the porphyrins are in register [12]. Alternatively, the

hierarchical self-organization of either the free base or the zinc nonamers into columnar 1-10 nm tall aggregates

results in nanoscaled materials with unique properties compared to individual porphyrins, covalent oligomers, large

aggregates, or macroscopic materials such as crystals and polymers. The self-organization of nonamers is a

secondary process driven largely by pi-stacking and electrostatics. The size of the aggregates is determined by the

minimization of particle surface-solvent interactions as is well known for good old-fashioned colloids. The major

difference is that each porphyrin in a layer is precisely ordered, while the layers are not ordered with respect to each

other. Both of these structural features are important for the photophysical properties of the system, vide infra [13,

14].

There are several ways to influence the average length of the columnar aggregates: (1) nature of the R-group

decorating the outer edges of the nonamer, (2) metalation state of the porphyrins, (3) nature of the surface the

aggregates are deposited on, and (4) it is also possible to trap larger aggregates out of the equilibrating solution.

Though this last method traps the kinetic rather than the thermodynamic products, once isolated and dried on a

surface, the structural integrity and photophysical properties of the aggregates are stable for years [13,14]. Both the

edge-to-edge interactions in the nonamer, and the face-to-face interactions between layers dictate the properties of

the aggregates. Though the heavy atom effect of the Pd(II) quenches the fluorescence of the free base porphyrins by

coordination chemistry in the nonamer, and by contact in the aggregates, the aggregates remain luminescent [12].

The addition of 1 to 4 equivalents of a Pd(II) compound with one labile ligand to tetrapyridylporphyrin results in

~67, ~45, ~30, and ~20 % of the observed fluorescence of the un-ligated porphyrin, respectively. This near

exponential decrease is what is somewhat higher than expected because of the fact that the heavy atoms are

coordinated to the exocyclic pyridyl groups rather than interacting with the porphyrin directly. Additionally,

porphyrin fluorescence is well known to be quenched in large aggregates due to a variety of factors such as shading,

energy transfer and concomitant branching kinetics, etc. Thus, the luminescence of the nonamer should be much less

than the ~ 36% of the collection of un-assembled porphyrins from a statistical estimation of the heavy atom effect.

The luminescence is observed; however, to be about 45% of the un-assembled porphyrins and the bands about 10%

narrower than expected. These preliminary results indicate that the chromophores in the aggregates are behaving

coherently as manifested by the luminescent blinking of individual aggregates. The intensity of the emitted light is

constant, but the duration of the ‘on’ state is not and ranges from less than a second to about a minute! (D. Adams,

C.M. Drain, et al, unpublished results)

In another functional mode, metalation of the nonamer with Co(II) (either in situ or using the porphyrinato cobalt

starting materials) results in a system with nine paramagnetic centers. The self-organization cobalt nonamer into 5-

10 nm tall aggregates is somewhat larger than observed for the free base, but can be understood in terms of the

increase proclivity for pi-stacking and other electrostatic interactions. As observed in the optical spectra of the

parent free base nonamer, in this material the high spin cobalt centers are electronically coupled – albeit weekly –

via both the horizontal pyridyl-paladium-pyridyl linkers and the vertical electrostatic interactions. Preliminary

magnetic force microscopy results show that the aggregates are indeed magnetic [13,14], but further studies

currently underway are required to characterize these properties.

Described below is an example of a functional photo transistor, figure 2, that uses all four levels of self-

organization [7, 15]. The synthesis of zinc porphyrin molecules appended with H-bonding or exocyclic ligands

represents the primary structure. The self-assembly of these into linear supramolecular arrays is the secondary

structure. These self-assembled tapes can be placed in an 8 nm thick by ~1 mm2 lipid bilayer separating two

compartments with 0.1 M salt solutions where they orient perpendicular to the bilayer-water interface and can adjust

their length to the width of the bilayer. The orientation and length adjustment in the bilayer is an example of tertiary

organization of matter into a material. The quaternary structure, in terms of a functional device, results when an

electron donor (source) is placed on one side of the bilayer, an acceptor on the other side [7,15,16], and calomel

electrodes connected to an operational amplifier are placed in the salt solutions on each side of the membrane. In

this device white light (or monochromatic light that corresponds to the porphyrin absorption bands) serves as the

gate. Thus, when the membrane device is illuminated a photocurrent is observed. The photocurrent depends, of

course, on the light intensity, the concentration of self-assembled wires, the dynamics of the membrane, and the

diffusion of the donor and acceptor molecules to the bilayer interface. This functional device depends on

nanoscaled supramolecular systems organized into a liquid crystal. It demonstrates that the interconnects to

nanoscaled devices need not be nanoscaled a priori – as in biology the Grotthus mechanism can be exploited in

water, polymers, etc.

APPLICATIONS OF SELF-ASSEMBLED PORPHYRIN SQUARES: CATALYSTS AND

SENSORS

Molecular squares are representative of a particular type of self-assembly that has been termed “directed

assembly”. Transition-metal based coordination complexes bias or direct the initial stages of the supramolecular

synthesis by selectively presenting ligation sites oriented at pre-defined angles with respect to each other [17]. Other

factors that can contribute significantly to the efficacy of directed assembly include metal-ligand bond lability [18]

and good molecular intermediate solubility [19]. Drain and Lehn have described the directed assembly of a flat

porphyrin square featuring L-shaped ligands (5,10-pyridylporphyrin species) and 180o-substituted PtCl2 linkers [8].

Stang and co-workers have described the formation of an octa-cationic porphyrin square nominally shaped as an

open-ended box [20]. This assembly features linear difunctional ligands (5,15-pyridylporphyrins) as walls and uses

diphosphine-chelated Pt(II) atoms as 90o connecters at the seams of the box. Slone and Hupp have reported on the

assembly of a box-like porphyrin square via reaction of Re(CO)5Cl with a linear porphyrin ligand (Figure 3) [21].

As shown in the figure’s scheme, the strong trans labilizing effect of CO leads to selective loss of a pair of carbonyls

initially ligated cis to each other. This opens up, at each Re center, two coordination sites oriented at ~90o with

respect to each other. Progressive coordination of the cis sites with difunctional porphyrin ligands generates a series

of right-angle-containing open assemblies that stay in solution because of weak coordination of solvent molecules at

remaining sites. Completion of a square and elimination of solvent molecules as ligands is accompanied by

precipitation. Among the consequences of this assembly sequence are: 1) high yields, as the equilibrium is pushed

toward square formation by removal of the product, and 2) mistake correction via dissociation of undesired open-

chain assemblies (soluble assemblies) and re-reaction until closed cycles (squares) are formed.

Multi-metallic porphyrin squares prepared according to Figure 3 have several useful properties. First,

despite the presence of Re they display significant photo excited state lifetimes and remain reasonably luminescence

[21]. Second, because rhenium is used in oxidation state I (resulting in stable 18-electron organometallic centers) the

squares are electrically neutral. This renders them insoluble in aqueous solutions – an important consideration in

many thin-film applications. Third, depending on the porphyrin used, the ligand walls define a dee p cavity having

edge lengths of ca. 18 to 22 angstroms [21,22]. Because the squares lack counter ions, in the solid state the cavities

initially contain only solvent molecules. Notably, coordinate-covalent bonds are sufficiently strong (i.e. 4 to 10

times stronger than typical hydrogen bonds) that squares remain intact and cavities persist even after removal of

solvent. Fourth, the octahedral coordination geometry of Re(I) offers the possibility of elaboration of the assemblies

in the direction normal to the plane of the square – for example, by replacement of chloride ligands with any number

of acetylide or di-acetylide terminated moieties.

These properties suggest applications that can capitalize on: a) cavity-containing squares as protective

encapsulants for delicate catalysts or as molecular hosts for analytes to be sensed, b) aggregates of squares as

selectively porous molecular materials, and c) isolated squares or ultrathin layers of squares as photochemical

sensitizers for redox and energy conversion processes. In our initial studies of supramolecular catalysis, we have

observed that encapuslation of an epoxidation catalyst (a manganese-containing di- or tetrapyridylporphyrin) by a

porphyrin molecular square (1) to yield assemblies 2 or 3, extends the lifetime of the catalyst from about 50

turnovers to more than 20,000 [23]. The observation is reminiscent of the catalyst stabilization achieved with

strapped porphyrins and other covalent architectures for active-site protection [24]. A potentially important

difference, however, is that the supramolecular assembly is modular: catalysts can be exchanged in and out, and

auxiliary sites within the square can be reversibly ligated to alter cavity size, chirality and chemical affinity [23,25]

The functional similarity of the supramolecular catalyst assembly to certain metalloenzymes has been noted: Both

types of assemblies employ potent, but indiscriminate catalysts. Selectivity is instead imparted by the surrounding

structure – either a tailored square cavity or a protein. In addition, the surroundings serve to prevent catalysts from

directly encountering each other and destructively interacting. Extension of the encapsulation idea to arrays of

squares comprising substrate-permeable thin films has resulted in enhanced substrate size discrimination and

dramatic increases in catalyst lifetime; turnover numbers of greater than 106 have been encountered with the limiting

factor evidently being the patience of the experimenter rather than the stability of the catalyst.

The existence of well-defined cavities and the comparative ease of preparing low defect films by casting

squares from volatile solvents can be exploited for nanoscale sieving. For films prepared on macroporous platforms

such as polyester membranes the sieving behavior is easily observed by positioning the membranes as separators in

U-cells and monitoring the selective appearance of appropriately sized dye molecules in receiving solutions [26].

Alternatively, by using redox-active probe molecules spanning a wide range of sizes, selective transport can be

followed electrochemically [27,28]. These experiments show that size cutoffs can be rationally manipulated by

using cavity-functionalized assemblies similar to 2 and 3. The experiments also show that transport rates vary

inversely with molecular film thickness, as expected if film-based diffusion rather than solution-to-film partitioning

is the rate-limiting process [27].

For many applications, including separations and catalysis, high molecular fluxes are desirable – something that

can be achieved by constructing very thin films. Our experience with molecular films indicates that significant

defect-based transport is very difficult to exclude for thicknesses of less than 20 nm. An effective alternative film

fabrication strategy uses phenyl-phosphonate functionalized porphyrin squares [29]. By positioning phosphonates at

the 10 and 20 positions of the parent porphyrin and square-forming ethynylpyridyl groups at the 5 and 15 positions,

we have obtained tetrarhenium squares featuring eight phosphonates – four directed above the plane of the square

and four directed below. Advantage can then be taken of the high affinity of phosphonates for Zr(IV) to carry out a

layer-by-layer assembly [30] of porous square films; Figure 4. Polarized spectroscopy establishes that films prepared

in this way are highly oriented with respect to the surface normal, while AFM measurements of micropatterned

films of the parent porphyrin show that film thicknesses can be controlled with extraordinary precision (single

molecular layer precision) [31]. Notably, the assembly methodology is sufficiently quantitative that efficient

electrochemical sieving can be demonstrated with a single layer. With size exclusion well documented, the obvious

next step is to engender selective molecular transport based on chemical properties. In principle, one way of

accomplishing this would be to attach phosphonate-terminated biopolymers or other moieties to the film exterior via

Zr(IV). Toward that goal, charge-selective transport has been demonstrated by capping porous porphyrin-

phosphonate films with hexaphosphonated coordination complexes having overall 10- charges.

Proof-of-concept studies of porphyrin squares as hosts for molecular or atomic (ionic) guests in chemical

sensing studies have focused on two problems: selective recognition of candidate guests and sensitive readout of

successful recognition events. The first has been addressed by functionalizing square interiors with appropriate

receptors, as shown in Figure 5 [25]. More than 100 cavity variants, including several chiral variants, have been

prepared in this way. In illustrative experiments, modest selectivity for alkali metal ions, Zn2+, and molecular iodine

as guest species has been observed [32,33]. An alternative approach that is a focus of current work is to gate the

entry of guests into extended square channels by appropriately functionalizing channel termini, rather than cavities

(channel interiors). This strategy, which can be usefully coupled to related membrane transport and separations

problems, has become increasingly attractive as our understanding of how to organize squares as highly oriented

thin-film materials has advanced.

The second part of the sensing problem – readout of successful binding by squares – was initially addressed by

focusing on the fluorescence of the square and its modulation by guests [32]. The approach proved only partially

successful, illustrating the need for a more general readout method. An intriguing alternative has been demonstrated

based on micropatterning porphyrin-square films; see Figure 6 [33]. These interesting structures efficiently diffract

coherent light – for example, from a laser pointer – to yield a characteristic diffraction pattern. The efficiency of the

diffraction process depends on the degree of refractive index contrast between the porous porphyrin film and the

surrounding medium (for example, water or air). Uptake of guest molecules increases the average refractive index of

the film and increases the diffraction efficiency, a quantity that is easy to evaluate experimentally. The attraction of

the method is that it is universal; the only requirement for modulation of index contrast and detection of uptake is

that the guest species contain electrons, a condition satisfied by all molecules. A useful variant of the method is one

in which the incident light is resonant with an allowed electronic transition of the porous material. Under these

conditions, signals can be selectively amplified by as much as three and one half orders of magnitude, greatly

improving the sensitivity of the technique while also imparting substantial chemical selectivity at the readout stage

[34]. An intriguing extension of the resonance strategy would be to apply it to the multi-chromophoric

metalloporphyrin arrays developed by Suslick and co-workers and described further in a following section [35].

Molecular square 1 has a B-band extinction coefficient of about 1.1 x 106 M-1cm-1, suggesting utility in

fundamental energy-transfer studies and in light-harvesting applications. The ability of the square to function as a

simple antenna has been noted [21]. Absorbed energy is rapidly transferred to a fifth porphyrin – a

dipyridylporphyrin attached to two of the four available Zn(II) sites within the square cavity – where it is re-emitted.

Preliminary studies of a related porphyrin square show that it readily sensitizes TiO2 to visible-light absorption and

electricity generation in a Grätzel-type [36] photoelectrochemical cell. The use of porous molecular materials could

obviate some significant ion transport problems that plague most multilayer sensitization schemes. In any case, the

square is attached to the photoelectrode surface via four of eight available phosphonate groups. The remaining four

can be used to bind a second layer of square via the zirconium chemistry described above. Repetition leads to build

up of additional layers and, of course, a systematic increase in film extinction. Curiously, however, additional layers

beyond the first two or three layers do not appreciably increase photocurrents. Electronic structure calculations

suggest that the effect (or lack of effect) with this first-generation sensitizer is a consequence of orientation of the

transition dipole moment for the lowest Q transition in a direction parallel rather than perpendicular to the electrode

surface. Thus, efficient lateral energy transfer (column to column EnT) should occur, but at the expense of vertical

energy transfer (EnT along the length of a square column). An obvious design objective for second-generation

sensitizers is to align transition dipole moments for the lowest energy transitions perpendicular to the surface.

Another will be to elaborate on the chromophoric properties so that broader spectral coverage can be obtained.

MULTICHROMOPHORIC SYSTEMS AS COMPONENTS OF PHOTONIC DEVICES

The successful development of new molecule-sized electronics promises to yield dramatic improvements in

component density, response speed, and energy efficiency. Given that energy and electron transfer processes within

molecules can take place on a sub-picosecond time scale, it is possible in principle to produce devices that function

far more rapidly than current devices. We have developed three fundamental approaches to producing molecular

switches based on photoinduced electron transfer. One of these strategies modifies the charge transmission

characteristics of the bridge molecule in a covalently linked donor-bridge-acceptor system on a picosecond time

scale [37]. The second strategy uses photoinduced electron transfer to produce a radical ion pair in the donor-

acceptor array: D+-A1--A2. The reduced acceptor A1

- possesses an intense optical absorption that can be irradiated

with a second laser pulse to produce the excited state D+-A1*--A2 that is capable of transferring an electron to the

secondary acceptor A2. We have already demonstrated that sequential application of femtosecond laser pulses to

both linear [38] and branched [39,40] systems of this type can propagate the electron up a potential gradient in the

linear case, and result in an optically controlled change of electron transport direction in the branched case. The third

approach recognizes that the rates of electron transfer reactions can be controlled through the application of electric

fields. The electric field produced by a photogenerated ion pair can have a large effect on the electronic states of

surrounding molecules. For example, a photogenerated electric field can affect the optical properties of a nearby

molecule, or even alter the rate of electron transfer between a second donor-acceptor pair. If each of these processes

is photodriven, logical devices can be developed based on the number, sequence, and frequencies of optical pulses

used to produce a particular optical observable in these systems.

In the past two years we have developed porphyrin-based molecular systems in which a photogenerated ion pair

produces a 106-107 V cm-1 electric field that electrochromically shifts a strong charge transfer optical transition in a

neighboring molecule modifying the optical transparency of a sample at a particular set of wavelengths. The overall

molecular design of these systems places a molecule that can be influenced by the photogenerated electric field at a

fixed distance and orientation relative to the ion pair. In this manner both the magnitude and direction of the electric

field are fixed relative to the orientation of the probe molecule. The photogenerated electric field significantly

perturbs the electronic structure of a probe molecule having a strong charge transfer transition. This perturbation

can be used to carry out switching operations in a repetitive manner on a picosecond time scale.

Molecule 4 has a 9-aminoperylene-3,4-dicarboximide chromophore (C) attached to a zinc 5-phenyl-10,15,20-

tripentylporphyrin donor – pyromellitimide acceptor pair (D-A), which undergoes quantitative photoinduced

electron transfer to form D+-A-. Chromophore C exhibits a broad charge transfer band at 535 nm [41]. Selective

excitation of C within D-A-C using 530 nm, 130 fs laser pulses produces 1*C, which undergoes singlet-singlet

energy transfer to produce 1*D, which in turn transfers an electron to A. If the D-A-C system is selectively excited

with 416 nm, 130 fs laser pulses to produce D+-A--C prior to excitation of C with 530 nm, 130 fs laser pulses, a 25%

lower yield of 1*C is generated. The intense local electric field produced by D+-A- causes a 15 nm electrochromic

red shift of the charge transfer absorption of C. Thus, the absorption of C at 530 nm is significantly diminished by

the presence of D+-A-. The need to use two laser pulses with different wavelengths to observe these effects, and the

resulting picosecond time response makes it possible to consider applications of this concept in the design of

molecular switches.

We are currently exploring modifications to this approach that utilize electric field induced wavelength shifts in

fluorescence emission as observables. This should make it possible to produce solid state arrays of these molecules

that can be addressed by light pulses with sub-diffraction-limit spatial extent. In this manner it may be possible to

produce high-density information processing arrays using this approach. The electrochromic shifts observed in

these systems can also be used to develop amplifiers to carry out all optical amplification over a broad range of

wavelengths. In these systems the photogenerated electric field effect of D+-A- is used to gate the transmission of an

intense light beam that is normally absorbed by the C chromophore. Ultrafast electrochromic shifting of the

absorption band of C results in a large increase in transmission of the second light beam. We are currently exploring

amplifier materials over the entire visible wavelength range, as well as optimizing the electronic structures of the

chromophores to give large electrochromic wavelength shifts.

A critical step towards photofunctional devices is the ability to create increasingly larger arrays of interactive

molecules. Covalent synthesis of large molecular arrays is highly inefficient and costly, thus making self-assembly

the method of choice to achieve ordered architectures from functional building blocks. Self-assembly can be based

on a variety of weak interactions such as hydrogen bonding and π-π van der Waals interactions as well as covalent

bonds. Photosynthesis is an important model for efficient photochemical energy conversion. In contrast to stepwise

charge separation in photosynthetic proteins, photoconversion using dye-sensitized semiconductors depends on

ultrafast electron injection from the photoexcited dye into the conduction band of the semiconductor followed by

rapid charge transport throughout the semiconductor structure.

We have studied a zinc 5,10,15,20-tetrakis(perylenediimide) porphyrin, ZnTPP-PDI4, that self-assembles into

ordered photofunctional organic nanoparticles using van der Waals interactions between the individual molecules

and exhibits not only the characteristics of an antenna-reaction center complex described above, but demonstrates

more extensive charge transport within the nanoparticles as well [42]. The ZnTPP-PDI4 molecule self-assembles

into ordered nanoparticles both in solution and in the solid state driven by van der Waals stacking of the PDI

molecules. The optical spectra of the (ZnTPP-PDI4)n nanoparticles strongly support the proposed structure depicted

schematically in Figure 7.

The PDI molecules most likely stack directly on top of one another in register, presumably at a van der Waals

contact distance of about 3.5 Å, while the ZnTPP molecules occupy sites in every other layer with an interlayer Zn-

Zn distance of about 7 Å. Photoexcitation of the nanoparticles results in quantitative charge separation in 3.2 ps to

form ZnTPP+ PDI- radical ion pairs in which the radical anion rapidly migrates to stacked PDI molecules that are on

average 5 layers removed from the site of their generation as evidenced by magnetic field effects on the yield of the

PDI triplet state that results from radical ion pair recombination. These field effects also show that the electron

hopping rate within the PDI stacks is about 2 x 1010 s-1. These nanoparticles exhibit charge transport properties that

combine important features from both photosynthetic and semiconductor photoconversion systems.

SENSOR ARRAYS BASED ON METALLOPORPHYRINS

The use of arrays of cross-reactive sensors has become a research area of intense interest over the past decade

[43]. Previous sensor arrays have employed a variety of chemical interaction strategies, including the use of

conductive polymers [44], conductive polymer/carbon black composites [45], fluorescent dye/polymer systems [46],

and polymer coated surface acoustic wave (SAW) devices [47]. In all such sensor arrays, however, the fundamental

interaction between the sensor components and the analytes has been weak physical adsorption, which has badly

limited the detection sensitivity. Suslick and coworkers [2, 35, 48] have developed a new sensor array technique

that makes heavy use of the ligation of analytes to metalloporphyrins in which colorimetric changes in an array of

dyes constitute a signal much like that generated by the mammalian olfaction system; each dye is a cross-responsive

sensor.

This technology uses a disposable two-dimensional array of chemoresponsive dyes as the primary sensor

elements, making it particularly suitable for detecting many of the odiferous compounds produced by bacteria.

Striking visual identification of a wide range of volatile organic compounds (VOC’s), including carboxylic acids,

alcohols, amines, ethers, thioethers, and thiols, are easily made at part per billion (ppb) levels (i.e., sensitivities

comparable to or better than gas chromatographic flame ionization (GC-FID) or mass spectrometric (GC-MS)

detection).

Design of the Colorimetric Sensor Array

The design of the colorimetric sensor array is based on two fundamental requirements: (1) the chemo-responsive

dye must contain a center to interact strongly with analytes, and (2) this interaction center must be strongly coupled

to an intense chromophore. The first requirement implies that the interaction must not be simple physical

adsorption, but rather must involve bond formation, strong acid-base interactions, or strong dipolar interactions. The

consequent dye classes from these requirements are (1) Lewis acid dyes (i.e., metal ion containing dyes), (2)

Bronsted acidic or basic dyes (i.e., pH indicators), and (3) dyes with large permanent dipoles (i.e., zwitterionic

solvatochromic dyes) (Figure 8).

Metalloporphyrins are a natural choice for the detection of metal-ligating vapors because of their open

coordination sites for axial ligation, their large spectral shifts upon ligand binding, and their intense coloration.

Metalloporphyrins have been previously employed for optical detection of gases such as oxygen [49] and ammonia

[50], and for vapor detection as chemically interactive layers on quartz crystal microbalances [51].

The large color changes induced in metalloporphyrins upon ligand binding creates a sensitive colorimetric

technique that minimizes the need for extensive signal transduction hardware. This represents the first example of a

colorimetric array detector for vapor phase ligands and makes it easy to obtain unique color change signatures for

analytes for both qualitative recognition and quantitative analysis by simply taking the difference before and after

exposure of scanned images of the array

The large spectral changes (and readily observable color changes) that occur in solution during ligand binding to

metalloporphyrins have been well documented. Solution studies have indicated that the magnitudes of spectral

shifts correlate with the polarizability of the ligand; hence, there exists an electronic basis for analyte distinction.

Furthermore, the periphery of the metalloporphyrin can be easily modified, thereby adjusting the accessibility of the

metal ion to the ligand and inducing shape-selective ligation. Using metal centers that span a range of chemical

hardness and ligand binding affinity and substituents that allow varying access to the metal, a wide range of volatile

analytes are differentiable. Porphyrins also show significant solvatochromic effects, so even weakly interacting

vapors (e.g., arenes, halocarbons, or ketones) show distinguishable colorimetric effects.

To image the arrays, a glass and Teflon cell holds the array in a fixed position on an inexpensive flatbed scanner

while gas containing the analyte(s) of interest flow over the array. The analyte mixture is prepared quantitatively

using digital mass flow controllers for serial dilution of commercial permeation tube sources or from saturation

towers at carefully thermostated temperatures. Alternatively, a portable hand-held prototype makes use of off-the-

shelf inexpensive commercial products: a white LED light source, a micro-air pump, and an ordinary web-camera

interfaced to a computer.

With a 6x6 array, as currently finalized, each analyte is represented as a 108-dimensional vector (36 red, green,

and blue values) each of which can take on one of 256 possible values (for inexpensive 8 bit digital cameras). The

theoretical limit of discrimination, then, would be the number of possible patterns, i.e., (256)108. Realistically,

however, the RGB vector components do not range over the full 256 possible values; R, G, and B values, however,

do vary over a range of 40. To discriminate patterns, let us assume a change of at least 4 is needed in the R, G, or B

value (actual discrimination is usually possible with only a change of 2). From principal component analysis, not all

of the 108 dimensions are equally important. In fact, roughly 95% of all information is contained in ~15 specific

dimensions (i.e., linear combinations of the 108 different R, G, and B values). This implies a ‘practical’ limit of

discrimination that is still immensely large: (40/4)15 = 1015 distinct patterns should be recognizable in a simple 6x6

array. For a translation of this expectation into chemical terms, the color change profiles of even closely related

compounds are distinct. For example, n-butylamine, n-hexylamine, t-butylamine, cyclohexylamine are all

differentiable from one another by simple examination of the pattern.

Analysis of Array Responses

No detailed statistical manipulation is necessary to distinguish among the patterns of different analytes, even

when they are closely chemically related. The color fingerprint for a given exposure is achieved by simply

differencing the pre- and post-exposure images. This can be accomplished using commercially available software,

e.g., Adobe Photoshop, and compared to a library of image data. An example of this differencing is shown in Figure

9.

In order to demonstrate the generality of this sensing technique, a wide range of chemical functionalities have

been examined and a comparison of color changes at saturation is shown in Figure 10. Each analyte is easily

distinguished from the others, and there are family resemblances among chemically similar species. Analyte

distinction originates both in the metal-specific ligation affinities and in their specific, unique color changes upon

ligation. Chemometric statistical tools have been used to study the porphyrin array responses. Principal component

analysis (PCA) studies revealed that the porphyrin array responses are relatively specific, having a relatively low

degree of redundancy. Furthermore, almost all of the chosen metalloporphyrins contribute to analyte distinction,

meaning the initial array was well-chosen for the vapor sensing task. Hierarchical cluster analysis (HCA) was used

to quantitatively compare vapor fingerprints. The HCA analysis revealed distinction of all of the tested vapors, with

groupings formed among similar analytes, such as phosphorus-containing ligands, sulfur-based ligands, and

nitrogenous bases. This is unique among electronic nose technologies and means that even in cases where the color

change pattern of an unknown analyte is not in the library, one will be able to tell what that analyte most closely

resembles in its chemical properties.

The color changes seen in the porphyrin array upon vapor exposure are attributable to ligation of the metal center

by the analyte. Diffuse reflectance spectra were obtained for single porphyrin spots both before and after exposure

to analyte vapors. Spectral shifts upon analyte exposure correlated well with those seen from solution ligation. For

example, Zn(TPP) exposure to ethanol and pyridine vapor gave shifts very similar to those resulting from ligation in

methylene chloride solutions containing ethanol and pyridine, respectively.

Further selectivity has recently been incorporated into the array by the use of sterically crowded

metalloporphyrins. Sen and Suslick synthesized a new class of bis-pocketed siloxyl-metalloporphyrins in which

extreme steric hindrance has been generated on both faces of 5,10,15,20-tetraarylporphyrinatozinc(II) complexes

using six, seven, or eight t-butyldimethylsiloxyl groups in the ortho-positions [52]. The bulky siloxyl groups were

appended to 5-phenyl-10,15,20-tris(2/,6/-dihydroxyphenyl)porphyrinatozinc(II) and 5,10,15,20-tetrakis(2/,6/--

dihydroxyphenyl)porphyrinatozinc(II) to produce highly protected metal sites. These porphyrins display remarkable

size and shape-selective ligation of a series of small vaporous organic amines, as shown in Figure 11. All three

porphyrins respond to the sterically unencumbered hexylamine, but none respond to the more sterically hindered

dipropylamine. The array is even able to differentiate between the very similar pyridine and piperidine; the

heptasubstituted TPP shows a greater response for pyridine than for the bulkier piperidine.

Array Sensitivities and Interference

This array detection relies on strong analyte-sensor interactions. Metal-ligand (i.e., metal-analyte) bonds range in

their bond enthalpies from ~40 to ~200 kJ/mol. In non-coordinating solvents (e.g., alkanes), equilibrium binding

constants are often >106 M-1. For pyridine, the vapor pressure is 0.02 atm at room temperature, so the Raoult’s

constant is ~2 x 10-3 atm M-1. For a binding constant of ~106 M-1, this is equivalent to ~2 ppb vapor! In contrast, the

enthalpy of physical adsorption (e.g., into polymers) is only ~5 to 20 kJ/mol (i.e., roughly a tenth of a metal bond).

Therefore, the equilibrium constant for adsorption will typically be only about 5 x 10-5 as large as that for ligation to

metal ions. Therefore, ligation is intrinsically ~20,000-fold more sensitive than adsorption into polymers.

Differences in the sensitivity of detection techniques, of course, can either enhance or diminish this intrinsic

advantage of ligation over adsorption.

An example of this extreme sensitivity is shown in Figure 12, which displays the array response to many of the

volatile gases commonly released by bacteria.

Changes in humidity are a very serious interference in other electronic nose technology. By using hydrophobic

dyes on a hydrophobic substrate, this problem has been eliminated. PVDF membranes of the type used routinely for

Western Blots provide excellent good dispersion of the chemoresponsive dyes on the surface, does not interact with

the metal atom, and is reflective enough to provide good signal for both scanning and diffuse reflectance

spectroscopy. Its hydrophobic nature also prevents interference from water vapor; a color fingerprint generated

from exposure of the array to n-hexylamine vapor with 0% relative humidity was identical to that with 100%

relative humidity. The insensitivity of this technology gives it a substantial advantage over polymer-based

electronic noses, all of which are susceptible to fluctuations in humidity.

OUTLOOK FOR THE FUTURE

The applications and devices described above demonstrate the incredibly diverse potential applications of

porphyrins. The stability of the macrocycle is sufficient for many of these to make it into commercially viable

applications. Self-assembly greatly reduces the cost of synthesis of multiporphyrin or multichromophoric systems

by increasing the yields sometimes by several orders of magnitude compared to the equivalent covalent arrays – if

the latter are synthetically accessible at all. However, if crystals are needed the yield of usable materials must be

considered. Combinations of chromophores may yield materials with properties unobtainable with single

component systems. The application of stochastic methods to analytical chemistry is a burgeoning field, and the

multifaceted spectral response of porphyrins and metalloporphyrins to various analytes makes them ideal signal

transducers for sensors.

Acknowledgments. JTH gratefully acknowledges the technical and scientific contributions of his co-workers at Northwestern University, as well as the financial support of the National Science Foundation, the U. S. Dept. of Energy Separations and Analysis Program, and the U. S. Dept. of Energy Solar Photochemistry Program for his group’s work on sensing + catalysis, sieving, and sensitization, respectively.

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4"L" + 4"T" + 1"X" + 12 PdCl2(NCPh)2 + 9 M2+

N

N N

N

R

N

N

RN

NN

N

R

N

N

NN

NN

N

R

N

N

R

N

N N

N

N

N

N

RN

NN

N

N

N

N

NN

NN

N

N

N

N

R

N

N N

NN

N

RN

NN

NN

N

NN

NN

NN

N

R

R R R

= Pd = 2H+, Zn2+, Co2+, Ni2+, Mn2+ R = tBu, Me, H

ClCl

Cl

Cl

Cl

Cl

ClCl

Cl

Cl

Cl

Cl

ClCl

ClCl

Cl

Cl

ClCl

Cl

Cl

ClCl

L L

LL

T

+ TT

T

~ 6nm diameter x 6 nm tall
Fig. 1.

N

N N

NNNH

NH

O

O

NHN

HN

O

ON N

NNNHN

O

ONN

N NNHO

ON N

N

O

ON

N

O

O

H HN

N N

NN

NH

NH

O

O

N

HN

HN

O

O

N N

NNNHN

O

O

NN

O

O

HN

N N

NNNH

NH

O

O

NHN

HN

O

O

R

EE

A

A

A

D

D

D

AD
Fig. 2a.

Fig. 2b.

=

R e

C O

C l

O C C O O C

C O +

T H F / T o l u e n e

r e f l u x , N 2

R e C l C O

O C O C

R e C O C O

C O C l

R e C O

C O

C O

C l R e O C

C O

C l

O C

N

N

N

N

Zn

(CH2)3CH3H3C(H2C)3

H3C(H2C)3 (CH2)3CH3

NN

Re..Re 24ÅRe..Re 20Å

NZnN N

R

R

N

N

N

O

Re..Re 24Å

Fig. 3. Directed assembly of porphyrin squares from dipyridyl edge ligands and Re(CO)5Cl. The smaller square corresponds to

assembly 1.

Fig. 4. Low-resolution atomic force microscopy image of a micropatterned film square 1. The pattern periodicity is 10 microns

and the pattern height is ca. 200 to 250 nm.

==

24Å

PO O

O

ZrZrO

OPO

N N

NN

N

N

P OPO

O

OZn

N N

NN

N

N

P OPO

O

OZn

N

NN

NN N

PO

PO

O

O

Zn

N

NN

NN N

PO

PO

O

O

ZnRe

OC

COCl

CO

ReCO

OC Cl

CO

Re

OC

CO

Cl

CO

Re CO

OC

Cl

CO

O

OO

O

O

O

O

O
Fig. 5. Layer-by-layer porphyrin square film assembly scheme.

ReClCO

OCOC

ReCOCO

COCl

Re CO

CO

CO

ClReOCCO

Cl

OC

Zn

Zn

Zn ZnN

N

N

N

ReClCO

OCOC

ReCOCO

COCl

Re CO

CO

CO

ClReOCCO

Cl

OC

Zn

Zn

Zn ZnN

N

N

N

N

N

N

N

Zn

(CH2)3CH3H3C(H2C)3

H3C(H2C)3 (CH2)3CH3

NZn = N

= Tunable unit which alters binding pocket properties

= Secondary guest within modified cavity

N = Tunable unit which alters binding pocket properties

= Secondary guest within modified cavity

N

Fig. 6. Schematic representation of cavity functionalization scheme and application of the functionalized cavity selective guest

recognition.

(structures)

1 2 3

N

N

O

O

O

O O

O

N

N

O

O

O

O

O

ON

N

O

O

O

OO

O

N N

N

N

Zn

N

N

O

O

O

O

O

O

ZnTPP-PDI4

hνννν e-

NN

O

O

O

O

O

O

N

N N

NZn

hνννν e-hνννν e-

NN

O

O

O

O

O

O

N

N N

NZn

Fig. 7.

1.5 cm1.5 cm

Metallo-

porphyrins

Solvatochromic

Indicator Dyes

Shape SelectiveMPs

Metallo-

porphyrins

Solvatochromic

Indicator Dyes

Shape SelectiveMPs

PVDF membranePVDF membrane

N NN

NMN NN

NM

Fig. 8. The colorimetric cross-reactive vapor sensor array. The chemical structure of metalated 5,10,15,20-tetra-

phenylporphyrins, MTPP is shown. TPP-2 is a dianion capable of strongly binding most M(II) and M(III) metal

ions.

ethyl acetateethanol acetone triethylphosphite tributylphosphinehexanethiolethyl acetateethanol acetone triethylphosphite tributylphosphinehexanethiol

DMSOhexanoic acidpropionic acidacetic acid hexylaminedmf DMSOhexanoic acidpropionic acidacetic acid hexylaminedmf

chloroformmethylene chloride toluenebenzenedipropylsulfidethf chloroformmethylene chloride toluenebenzenedipropylsulfidethf

Fig. 10. Color fingerprints for a variety of analytes at saturation vapor pressure. These images result from

differencing the images collected before and after exposure to analyte vapor. (These images, of course, are much

more dramatic in color than in gray scale.)

R = Si(CH3)2(C(CH3)3) = tbdmsiOR’ = OR” = H Zn(Si6PP): largest pocket.

R’ = H R”= R Zn(Si7PP): small pocket one face,larger pocket other.

R’ = R” = R Zn(Si8PP): small pocket both faces.

N N

N NZn

OROR

RO

OR

OR”

RO

RO

R’O

N N

N NZn

OROR

RO

OR

OR”

RO

RO

R’O

N

PyridinePiperidine

NH

CyclohexylamineNH2

DiisopropylamineHN

HexylamineNH2

(Si)6 (Si)7 (Si)8

N

Pyridine

NN

PyridinePiperidine

NHPiperidine

NHNH

CyclohexylamineNH2

CyclohexylamineNH2NH2

DiisopropylamineHN

DiisopropylamineHNHN

HexylamineNH2

HexylamineNH2NH2

(Si)6 (Si)7 (Si)8(Si)6 (Si)7 (Si)8

Fig. 11. Left: Structure of shape-selective siloxylporphyrins. Right: Colorimetric response of shape selective

siloxylporphyrins to several electronically similar, but sterically different analytes. The rest of the array

distinguishes among 1°, 2°, 3°, and aromatic amines.

H2S14 ppb

H2S14 ppb

CH3SH22 ppbCH3SH22 ppb

n-hexylamine100 ppb

n-hexylamine100 ppb

Acetic acid170 ppb

Acetic acid170 ppb

Formic acid140 ppb

Formic acid140 ppb

3-Me-2-hexenoicacid: 12 ppb

3-Me-2-hexenoicacid: 12 ppb

Iso-valeric acid280 ppb

Iso-valeric acid280 ppb
Fig. 12. Part per billion responses to volatile vapors produced by bacterial growth.

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